Practical_Antenna_Handbook_0071639586

C h a p t e r 2 9 : T o w e r s 667
Figure 29.6 Safety precaution.
solution for some, but prospective owners might
wish to consider whether they want to be off the
air every time the wind gets above 50 mph or
whether they want to go outside and crank it up
every time they get the urge to sit down at the rig
and operate.
Steel safety pipes
IN ADDITION TO
normal fasteners
Self-ÂSupporting Towers
As mentioned earlier, self-Âsupporting towers may
be designed and constructed with a single fixed
diameter or cross section from top to bottom or
with a tapered cross section. The greater the taper,
the more constant (and smaller) the compressive
forces in the downwind leg or side of the tower
will be from top to bottom.
Tapered Cross-ÂSection Towers
Old-Âfashioned windmill towers seen on many
farms are an excellent example of the basic design
that governs the construction of self-Âsupporting towers. Stated very simply, the slope of
the sides of the tower determines the ratio of compressive force in the leeward tower
leg(s) at the bottom of the tower to the force exerted on the antenna (or windmill!) by
the wind. When there is very little taper (i.e., very little difference in tower cross-Â
sectional area between top and bottom), the multiplier is quite high; when the slope of
the sides or legs is pronounced, the multiplier is much lower. The value of sloping the
legs of a tower is best understood by considering the value of spreading your feet to
brace yourself against a strong wind.
Historically of primary use in commercial applications (and occasionally in residential
wind generator systems), the Rohn SSV series of tower sections has found favor
with increasing numbers of radio amateurs. The current family of 16 different sections—each
20 ft long—in principle allows for construction of towers up to 320 ft in
height. A heavy-Âduty version of the SSV family replaces the bottom eight sections with
beefed-Âup alternates. A shorter tower can be selected from many different sets of consecutive
sections, so if your objective is to build a 100-Âft self-Âsupporting tower, the SSV
gives you nearly two dozen options(!) to choose from, depending on the maximum
wind loading. The beauty of this is that you can customize the strength (and cost!) of
your tower based on the maximum wind load you anticipate.
Make no mistake about it—self-Âsupporting towers are expensive! Even though
there are no guy wire costs, the superior strength of the sections makes them more expensive
than guyed tower sections per foot of height, and the size of the concrete base
(which takes the place of the guy wires in resisting any tendency for the tower to tip
over in a strong wind) is massive. Further, installation of these 20-Âft sections of tower
will undoubtedly require professional rigging companies, and the interface between